It is known from Kinney, U.S. Pat. No. 3,338,992 to prepare loose fibrous webs by depositing well separated filaments upon a moving collecting surface in random array. Processes are also known for heat treating such webs to prepare bonded nonwoven sheets. When the webs are composed of polyester filaments, changes in the crystalline structure of the polymeric material occur, such changes in crystallinity being taught in Piccard et al. U.S. Pat. No. 2,836,576.
One known method of bonding is to expose the web to high pressure steam while the web is restrained by compressive forces applied by belts, screens or other devices. A process of this type is described in Levy, U.S. Pat. No. 3,276,944. The present process relates to bonding of mixtures of homopolymer filaments and copolymer filaments.
Another known method for bonding fibrous webs is to blow hot air through the web as in Krikorian, U.S. Pat. No. 3,417,925 or by use of flow-through paper drying machinery. By the latter method, nonwoven sheets have been prepared using a vacuum roll with a pervious honeycomb surface as described by E. T. Bryand in Paper Trade Journal, Mar. 15, 1971. The roll surface provides support for the fibrous web during bonding. Hot air is directed against the filamentary web to heat it and the air is pulled through the web by means of a vacuum applied from the inside of the pervious roll surface which also serves to hold the web against the roll.
The present process is particularly useful for bonding mixtures of polyethylene terephthalate homopolymer and copolymer filaments. It permits high speed consolidation of the web on the collection belt, transfer of the consolidated web, i.e., one which can support its own weight, to the bonder and bonding the web at reduced temperatures.